The mechanical probe is the principal instrument used in the past for making voltage measurements at the internal nodes of an IC. As IC density has increased, it has become increasingly difficult to employ the mechanical probe. The conductive lines in many current IC's are so fine that they are easily damaged by the mechanical probe. The instrument is often too large to contact a single line but avoid its neighbors. Also, today's measurements must be more accurate than in the past. The capacitive loading present when the mechanical probe contacts a conductive line can give a misleading result.
The E-beam probe generally overcomes these disadvantages. FIG. 1 illustrates the basic features of an E-beam probe system of the planar retarding field analyzer type. The operation of this system is described in: Feuerbaum, "VLSI Testing Using the Electron Probe," Scanning Electron Microscopy (SEM), 1979, pp. 285-296 and 318; Feuerbaum et al., "Improved Secondary Electron Signal Processing for Waveform Measurements, " SEM, 1982, pp. 1501-1505; Menzel et al., "Secondary Electron Analyzers for Voltage Measurements," SEM, 1983, pp. 65-75; and Feuerbaum, "Electron Beam Testing: Methods and Apllication," Scanning, Vol. 5, No. 1, 1983, pp. 14-24.
In brief, an E-beam source 10 directs a beam of primary electrons (PE) 12 toward a patterned electrically conductive layer 14 lying on an electrically insulating layer 16 of a device 18 such as an IC. As PE 12 impact the specific conductive portion being examined, they cause it to emit secondary electrons (SE). The energy of the SE depends on the voltage V.sub.A applied to the portion under examination.
The SE are analyzed with a spectrometer 20 which contains an extraction grid 22 maintained at a high voltage for accelerating the emitted SE, enabline them to overcome retarding fields near the upper surface of device 18. A retarding grid 24 decelerates the emitted SE and allows the SE 26 having a certain minimum energy to pass. A deflection grid 28 diverts SE 26 out of spectrometer 20 toward a detector 30. It collects SE 26 and generates an electrical signal representative of their energy.
Measurements are more accurate if the shape of the curve representing the number of emitted SE as a function of electron energy remains constant as voltage V.sub.A changes. This is achieved by arranging a feedback loop so that substantially the same number of SE 26 are collected regardless of the value of V.sub.A. In the feedback loop, a comparator 32 provides grid 24 with a voltage V.sub.R at a value sufficient to maintain the collected SE current substantially constant. Voltage V.sub.R is the output signal of the E-beam system. In the absence of measurement error, V.sub.R tracks V.sub.A and is typically linearized one-to-one so as to (ideally) equal V.sub.A.
Local electric fields resulting from neighboring portions of device 18 at voltages different than V.sub.A produce measurement errors. Microfields influence the trajectories of the emitted SE and consequently the detected SE current. If any of the microfields are of the retarding type, they also hinder low-energy SE from passing grid 24. Raising the voltage on grid 22 to increase the extraction field strength can improve the efficiency of SE collection under certain conditions. However, this is merely an avoidance technique. For many applications, it is desirable to understand quantitatively how local fields affect voltage VR so as to correct its value appropriately.
Nakamura et al. studied the quantitative influence of local fields in "An Analysis of the Local Field Effect on Electron Probe Voltage Measurements," SEM, 1983, pp. 1187-1195. FIG. 2 illustrates the basic features of their semiconductor test device. Layer 14 consisted of three sets 34, 36, and 38 of conductive lines. Each set consisted of a sample conductor LS flanked symmetrically by a pair of identical neighbors L.sub.N. The widths w.sub.S and w.sub.N of conductors L.sub.S and L.sub.N and the distance d.sub.N from conductor L.sub.S to either neighbor L.sub.N all varied from 10 to 50 micrometers. The PE beam energy was 25 kiloelectron volts (keV). As is evident from FIG. 2, Nakamura et al. determined how one of w.sub.s, w.sub.n, and d.sub.n affected voltage V.sub.R while the other two were held constant.
Nakamura et al. do provide a step forward in determining the quantitative effect of local fields. However, conductor widths and spacings of current IC interest are under 10 micrometers, typically 1-5 micrometers. This is about an order of magnitude less than the dimensions studied by Nakamura et al. Their 25-keV beam energy would cause significant damage to many present IC's. To avoid damage, the beam energy should be at least an order of magnitude lower. Attempting to scale their designs down to state-of-the-art dimensions and beam energies would be difficult, at best.
Furthermore, Nakamura et al. used an electrode arrangement in which the dimensions varied along the conductor lengths. Reproducing such a tapered geometry at current IC dimensions would be highly impractical. Significant measurement error would appear to exist in simply ascertaining the locations along the conductors where the measurements are taken. The work of Nakamura et al. is of academic interest but little practical utility to present/future IC technology.